The odd one out:Arabidopsisreticulon20 has a role in lipid biosynthesis

Abstract

The family of reticulon proteins has been shown to be involved in a variety of functions in eukaryotic cells including tubulation of the endoplasmic reticulum (ER), formation of cell plates and primary plasmodesmata. Reticulons are integral ER membrane proteins characterised by a reticulon homology domain comprising four transmembrane domains which results in the reticulons sitting in the membrane in a W-topology. Here we report on a subgroup of reticulons with an extended N-terminal domain and in particular on arabidopsis reticulon 20. We show that reticulon 20 is located in a unique punctate pattern on the ER membrane. Its closest homologue reticulon 19 labels the whole ER. We show that mutants in RTN20 or RTN19, respectively, display a significant change in sterol composition in the roots indicating a role in lipid biosynthesis or regulation. A third homologue in this family -3BETAHSD/D1-is localised to ER exit sites resulting in an intriguing location difference for the three proteins. IntroductionThe endoplasmic reticulum (ER) is a multifunctional organelle (Hawes et al., 2015) involved in a plethora of aspects of plant life. The polygonal network of the cortical ER consists of motile tubules that are capable of morphing into small cisternae, mainly at the three-way junctions of the ER network (Sparkes et al., 2009). The plant cortical ER network has been shown to play numerous roles in protein trafficking (Palade, 1975;Vitale and Denecke, 1999) and pathogen responses (Pattison and Amtmann, 2009; Beck et al., 2012). It is a highly dynamic organelle and previous studies have demonstrated a possible link between ER structure and function within different cell-and tissue-types (Stephenson and Hawes, 1986; Lai et al., 2014).A variety of ER movements have been characterised, including growth and shrinkage of tubules, rearrangement of the polygonal network (Sparkes et al., 2009), movement of the membrane surface (Runions et al., 2006), and the conversion between cisternal and tubular ER (Hawes et al., 2015). These distinct movements which appear dependent on the acto/myosin system (Sparkes et al. 2009) and the possibly significant link between structure and function, makes these processes important to understand. not peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was . http://dx.doi.org/10.1101/123679 doi: bioRxiv preprint first posted online 3 The reticulon protein family: Reticulons are a family of proteins found in a wide range of eukaryotes and have been shown to localise to the ER in many species, including mammals, yeasts and plants (Nziengui et al., 2007;Yang and Strittmatter, 2007).Previous studies have demonstrated a role for these proteins in moving and shaping the ER into tubules (Voeltz et al., 2006;Yang and Strittmatter, 2007; Tolley et al., 2008).Plant reticulons (RTNLB-reticulon-like protein B; henceforth referred to as RTN) are considered to be essential in maintaining the tubular ER network ...